DD,
A couple of thoughts, ducks don't usually eat fish so the connection might be a little more remote than a heron for example. Also, BG might not be a good example, if there is animal husbandry, they might start by building the food chain with a hardy forage fish. Maybe a certain species of fish lay eggs with a protective coat allowing them to pass through a certain bird.
In my neighborhood there are fish that walk from one pool to another. The more I pay attention the more I, at least think, I learn. Though, paying attention seems to get harder the older I get.
